The present invention relates to the field of toolholders for the computer numerically controlled (cnc) machining industry.
The typical method of controlling the depth that a cutting tool is able to plunge into a material being machined is by placing the cutting tool in a rigid toolholder and using the computer controlled axis to plunge the tool to a predetermined depth. The operator of the cnc machine programs the machine to plunge the tool to the desired depth. If the part is perfectly flat and the exact location of the top surface of the work-piece is known, then this method works very well. However, when the material is not perfectly flat or when the top surface of the work-piece is not precisely known, then this method is unacceptable.
Two specific processes, engraving and countersinking holes, are very susceptible to variations in the work-piece surface. If the work-piece surface is uneven and the above mentioned method of controlling depth is used to perform engraving, the width of the engraved line appears uneven and often leads to tool breakage due to the tool tip plunging too far into the surface of the material being engraved. If the above method is used to perform countersinking of holes, then the diameter of the countersunk hole being produced may be incorrect. The purpose of the countersunk hole is to allow a screw head to be perfectly flush with the work-piece surface. If the countersunk hole is too deep, the top of the screw will be below the work-piece surface and if the countersunk hole is too shallow, the top of the screw will be above the work-piece surface. The aerospace industry is one place where the diameter of the countersunk hole is critical. Screws are placed onto the surface of the wings to hold the sheetmetal skin surface to the frame. The screws need to be perfectly flush so they do not affect wind resistance.
Devices are available which limit the amount of the tool tip exposed to the work-piece. These devices are intended to be rotated in a spindle of a manually operated machine such as a drill press or hand drill where the operator controls when to stop the downward motion of the tool against the work-piece. These devices have a hard stop which limits the amount the spindle can be moved toward the work-piece. Instead of a cnc controller defining when to stop the downward motion of the spindle and the tool towards the work-piece, the hard stop contacts the surface of the work-piece and will not allow the spindle to be moved any closer to the work-piece.
Mercier U.S. Pat. No. 6,474,450, Zimmer U.S. Pat. No. 2,422,279, Swim, Jr. U.S. Pat. No. 5,066,172, Schmidt U.S. Pat. No. 2,216,988 and Schmidt U.S. Pat. No. 2,192,528 all describe devices that utilize hard stops to control the downward motion of the tool into the work-piece. None of these devices could be used in a cnc machine where the computer control determines when to stop the downward movement of the spindle towards the work-piece. If any of these devices were placed in the spindle of a cnc machine and plunged into the work-piece, the cnc machine would simply crush the hard stop against the work-piece surface and damage the tool, the workpiece, and possibly the cnc machine. The cnc machine would need to stop the downward motion of the tool before it reached the hard stop which would negate the use of the device in the first place.
Nelson U.S. Pat. No. 6,834,434 and Nelson U.S. Pat. No. 7,093,368 describe a Spring Loaded Engraving Tool and one with a Depth Controlling Nosepiece respectively, which effectively solves the problem of an irregular work-piece surface when engraving is performed using a cnc machine. A spring is used to press an engraving tool into the work-piece surface so it is not critical to know the precise location of the work-piece surface. To absolutely control the depth of engraving, a nosepiece may be attached to the front of the Spring Loaded Engraving Tool to only allow a specific length of the tool to protrude from the end. Then, when the tool is plunged down into the work-piece, the tool tip is only able to penetrate the work-piece surface by a specific amount. There is no hard stop limiting the distance the spindle can be plunged into the work-piece surface. The spring will simply compress further while the nosepiece remains biased against the work-piece.